Multithreading condition synchronization of variables

    Condition variables are another means of thread synchronization, the main logic is to wait and wake up. When the condition is not satisfied, thread waits; conditions are met, thread ( other threads ) wake. Condition variables are generally used in conjunction with a mutex, because of the need to ensure that multi-threaded mutex modified conditions.

Functions involved are:

int pthread_cond_init(pthread_cond_t *restrict cond,
const pthread_condattr_t *restrict attr);

int pthread_cond_destroy(pthread_cond_t *cond);

int pthread_cond_wait(pthread_cond_t *restrict cond,
pthread_mutex_t *restrict mutex); 

int pthread_cond_signal(pthread_cond_t *cond); 

int pthread_cond_broadcast(pthread_cond_t *cond);

Practical Training:

There are three threads print A, B, C, please use the multi-threaded programming, printing cycle 10 times ABCABC on the screen ...

solution：

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define N 10
#define M 3          //number of threads
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond;
int m = 0;

void* thr_fn(void *arg)
{
    int index = (int)arg;
    char ch = 'A' + index;
    int i;
    for (i = 0; i < N; ++i)
    {
         pthread_mutex_lock(&mutex);
         while (index != m)
             pthread_cond_wait(&cond, &mutex);

         printf("%c", ch);
         m = (m+1) % M;
         pthread_mutex_unlock(&mutex);

         pthread_cond_broadcast(&cond);
    }
    return (void*)0;
}

int main ()
{
    pthread_cond_init(&cond, NULL);
    pthread_t tid[M];
    int i;
    for (i = 0; i < M; ++i)
        pthread_create(&tid[i], NULL, thr_fn, (void*)i);
    for (i = 0; i < M; ++i)
        pthread_join(tid[i], NULL);
    pthread_cond_destroy(&cond);
    printf("\n");
    return 0;
}

 REFERENCE:

[Machine] Huawei 2014 test pilot school recruit machines: multi-threaded print cycle ten times ABC

Reproduced in: https: //www.cnblogs.com/gattaca/p/4732623.html